Field augmented permanent magnet structures

ABSTRACT

A permanent magnet comprising a shell of magnetic material having a hollowavity and an access port that passes through the shell and communicates with the cavity. The shell is permanently magnetized to produce a magnetic field in the cavity. A magnetic insert is located in the cavity. The insert has a tunnel aligned with the access port and is magnetized in a direction opposite to the direction of the magnetic field. Specifically, a spherical magnetic shell has a concentric cavity in which a spherical magnetic insert is housed. An access port in the form of an axial hole passes through the spherical center of the shell and the insert. The shell (&#34;magic sphere&#34;) is magnetized such that it is capable of producing a uniform magnetic field in the cavity. The insert is uniformly magnetized in a direction opposite to that of the cavity field produced by the shell. As such a working field that has a strength greater than that of the cavity field produced by the shell is located in the tunnel. Another embodiment shows a cylindrical shell having an access port in the form of a narrow, gap that passes through the cylindrical shell (&#34;magic ring&#34;) and a uniformly magnetized concentric insert. The insert augments the working field that is produced by the shell. An outer, uniformly magnetized shell that houses the structure may be tailored to cancel exterior dipole fields.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto me of any royalty thereon.

NOTICE OF CONTINUATION

This application is a continuation-in-part of application Ser. No.07/892,093, entitled FIELD AUGMENTATION IN HIGH-INTENSITY MAGNETIC FIELDSOURCES, by inventor Herbert A. Leupold, Attorney Docket No. CECOM 4665,filed Jun. 2, 1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high-field permanent magnets. Morespecifically, it relates to structures and techniques for augmenting aworking magnetic field contained in a cavity of a permanent magnet.

2. Description of the Prior Art

High-remanence, high-coercivity, permanent-magnet materials, such asthose of the rare-earth type, have improved so that it is now practicalto make flux sources of extraordinary strength and compaction. Examplesof high-intensity, compact permanent magnets may be found in thefollowing references:

Leupold, U.S. Pat. No. 4,835,506, entitled "Hollow SubstantiallyHemispherical Permanent Magnet High-Field Flux Source;"

Leupold, U.S. Pat. No. 4,837,542, entitled "Hollow SubstantiallyHemispherical Permanent Magnet High-Field Flux Source for Producing aUniform High Field;"

Leupold, U.S. Pat. No. 4,839,059, entitled "Clad Magic Ring Wigglers;"

Leupold, U.S. Pat. No. 5,103,200, entitled "High-Field Permanent MagnetFlux Source;"

Leupold, U.S. Statutory Invention Registration H591, entitled "Method ofManufacturing of a Magic Ring;"

Leupold et al., "Novel High-Field Permanent-Magnet Flux Sources," IEEETransactions on Magnetics, vol. MAG-23, No. 5, pp. 3628-3629, Sept.1987;

Leupold et al., "A Catalogue of Novel Permanent-Magnet Field Sources,"Paper No. W3.2, 9th International Workshop on Rare-Earth Magnets andTheir Applications, pp 109-123, Aug. 1987, Bad Soden, FRG;

Leupold et al., "Design applications of magnetic mirrors," Journal ofApplied Physics, 63(8), Apr. 15, 1988, pp. 3987-3988;

Leupold et al., "Applications of yokeless flux confinement," Journal ofApplied Physics, 64(10), Nov. 15, 1988, pp. 5994-5996; and

Abele et al., "A general method for flux confinement in permanent-magnetstructures," Journal of Applied Physics, 64(10), Nov. 15, 1988, pp.5988-5990.

Additionally, magnets of the type described herein may be found in thefollowing co-pending U.S. patent applications that are incorporatedherein by reference:

Ser. No. 654,476, filed Feb. 13, 1991, entitled "High-Power ElectricalMachinery;"

Ser. No. 650,845, filed Feb. 5, 1991, entitled "High-Power ElectricalMachinery with Toroidal Permanent Magnets;" and

Ser. No. 892,104, filed concurrently herewith, entitled "Magnetic FieldSources Having Non-Distorting Access Ports," Docket No.: CECOM 4666.

These references show a number of high-intensity permanent magnetshaving a variety of different compact shapes. In general, the magnetsdescribed in these references have a shell of magnetic material and acavity in which a working field is located. Access ports of varioussizes, shapes and locations pass through the shell and communicate withthe cavity.

Salient among these magnets are cylindrical ("magic ring") and spherical("magic sphere") magnetic shells in which the direction of magnetizationchanges as a function of a polar angle. These magnets produce in theircavities uniform, polar-axial transverse fields. Theoretically, there isno limit to the fields attainable in a cavity of this type if one iswilling to employ enough magnetic material of sufficiently highcoercivity to retain its magnetism in the face of the high distortingfields engendered by the structure.

In practice, it is difficult to produce a spherical or cylindrical shellhaving a remanence or remanent magnetization the direction of whichcontinuously varies. Consequently, such shells are typically constructedfrom segments that are each uniformly magnetized. When nested, thesegments form a magnetic shell. In the case of a segmented cylindricalshell, the angular direction of magnetization changes abruptly by 4 π/Nbetween adjacent segments, where N is the number of nested segments.

A working field produced by a segmented shell suffers surprisinglylittle from the approximation by segmentation. For example, if acylindrically shaped shell is divided into sixteen segments, it producesa magnetic field of over 97% of that produced by a continuous structure.Even with a coarse approximation of only eight segments, 90% of theideal field is realized. Specifically, a segmented spherical shellhaving an outer radius of 3.3 centimeters that is made of a magneticmaterial having a remanence of ten kilogauss can produce a field ofsixteen kilo-oerstead in a spherical cavity having a radius of only 1.0centimeter. The shell would have a mass of only 1.1 kilograms. Similarperformance is obtainable from cylindrical and hemispherical structures.

Although such high-intensity, compact magnets have served the purpose,those concerned with their development have recognized the need forstructures and techniques that further increase magnetic intensitywithout appreciably increasing size and mass. The present inventionfulfills this need.

SUMMARY OF THE INVENTION

The general purpose of this invention is to provide a compactpermanent-magnet flux source having a working field of very highintensity. To attain this, the present invention contemplates a uniquefield-enhancing insert of active (permanent magnet) or passive (iron)magnetic material that combines with a magnetic flux source to increasethe intensity of the working magnetic field.

Sometimes, as in certain fiber-optic or electron-beam applications, aworking field is needed only over a narrow region. However, in manymagnet structures, the space available for the working field isrelatively large. For example, in those flux sources that have amagnetic shell and a cavity, as described above, much of the cavityspace is often not used: a narrow cylindrical region around a polar orequatorial axis in the center of the cavity is all that is usuallyneeded for a working field. It is contemplated in the present inventionthat the unused or excess space in such structures be filled with afield-enhancing insert having active or passive magnetic properties soas to increase the intensity of the working field.

More specifically, the present invention includes a permanent magnetcomprising a shell of magnetic material having a hollow cavity and anaccess port that passes through the shell and communicates with thecavity. The shell is permanently magnetized to produce a magnetic fieldin the cavity. A field-enhancing insert having a tunnel bore hole islocated in the cavity such that the insert tunnel is aligned with theshell's access port. The field enhancing insert is permanentlymagnetized in a direction opposite to the direction of the cavity fieldif the access port and cavity are in-line with the polar axis of theshell (axial). The insert, however, is either a passive magnet (iron) ora permanent magnet magnetized in the same direction of the cavity fieldif the access port and cavity are perpendicular to the poles of theshell (equatorial).

According to another aspect of the invention, a spherical magnetic shellhas a concentric cavity in which a spherical magnetic insert is housed.An access port in the form of an axial hole passes through the sphericalcenter of the shell and the insert. The shell ("magic sphere") ismagnetized such that it is capable of producing a uniform magnetic fieldin the cavity. The field-enhancing insert is uniformly magnetized in adirection opposite to that of the cavity field produced by the shell. Assuch a working field that has a strength greater than that of the cavityfield produced by the shell is located in the axial hole.

According to still another aspect of the invention, the access port is anarrow, axial gap that passes through a cylindrical shell ("magic ring")and a uniformly magnetized concentric field-enhancing insert. The insertaugments the working field that is produced by the shell. An outer,uniformly magnetized shell can house the structure to cancel exteriordipole fields.

According to yet another aspect of the invention, a "magic sphere" shellhas a concentric cavity in which a spherical magnetic insert is housed.An access port in the form of an equatorial hole passes through thespherical center of the shell and the insert. The "magic sphere" shellis magnetized such that it is capable of producing a uniform magneticfield in the cavity. The field-enhancing insert is uniformly magnetizedin the same direction as the cavity field produced by the shell. Assuch, the working field has a much greater strength than the cavityfield alone.

According to still a further aspect of the invention, a "magic sphere"shell has a concentric cavity in which a passive magnet (iron) insert ishoused. An access port in the form of an equatorial plane is passedthrough the spherical center of the shell to essentially separate thesphere into two half-spheres. The shell of each half-sphere ismagnetized such that it is capable of producing a uniform magnetic fieldin the cavity and creating magnetic excitation in the insert. Theexcited passive magnet insert, in turn, augments that cavity fieldproduced by the shell. Moreover, if the "magic sphere" shell ismagnetized such that it saturates the passive magnet insert, the insertwill create maximum magnetic field augmentation in the cavity.

According to yet another aspect of the invention, the magic sphere abovehaving an access port passing through the spherical center of the shell,including the insert, in the form of a cylindrical tunnel rather than anequatorial plane. The shell is magnetized such that it saturates theinsert because the insert can not augment the field otherwise.

According to still another aspect of the invention, the access port is anarrow, equatorial gap that passes through a "magic ring" shell and auniformly magnetized concentric field-enhancing insert. Insertion of apassive or permanent magnet in the cavity augments the shell's workingfield. An outer, uniformly magnetized shell can house the structure tocancel exterior dipole fields.

The exact nature of this invention, as well as other objects andadvantages thereof, will be readily apparent from consideration of thefollowing specification relating to the annexed drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of a preferred embodiment of the inventionshowing an axial "magic-sphere" type magnet in cross section and with aninety-degree section removed.

FIG. 2 is a pictorial view of another embodiment of the inventionshowing an axial "magic-ring" type magnet.

FIG. 3 is a sectional elevation view of an alternate embodiment of thedevice shown in FIG. 1.

FIG. 4 is a cross sectional view of another embodiment of the inventionshowing half of an equatorial "magic sphere" type magnet having a planarhole bisecting the sphere into two half-spheres and an active magnetinsert.

FIG. 5 is a pictorial view of yet another embodiment of the inventionshowing an equatorial "magic ring" type magnet with an active magnetinsert.

FIG. 6 is a pictorial view of an alternate embodiment of the deviceshown in FIG. 5 showing an equatorial "magic ring" with a passive (iron)magnet insert.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, there is shown in FIG. 1 a high-fieldpermanent magnet 20 having a spherical magnetic shell 21, a concentricspherical cavity 22, and a spherical magnet insert 23 located in cavity22. Coaxial access ports 24 and 25 pass through shell 21 on oppositesides of cavity 22 along a polar axis. A tunnel 26, which passes throughthe spherical center of magnetic insert 23, is coaxially aligned withports 24 and 25. Ports 24 and 25 and tunnel 26 form a narrow,cylindrical hole of sufficient size to permit proper utilization of aworking field which is located in tunnel 26. Utilization of the workingfield in tunnel 26 would typically be by way of an optical fiber, anelectron beam, a waveguide, etc.

Magnetic shell 21 is formed as a conventional segmented "magic-sphere"type magnet. Shell 21 comprises a series of nested cones 31-39 of equalangular extent, i.e. each cone subtends an angle of twenty degrees, andis centered about a vertical polar axis, where the longitudinal axis ofports 24 and 25 and tunnel 26 is the vertical polar axis. Consideringcone 32, by way of example, it is readily seen to be a shell havinginner and outer spherical surfaces and nesting conical surfaces that arespaced by twenty degrees and intersect at the spherical center.

While nine cones are depicted in FIG. 1, magnet 20 might comprise afewer or larger number. Of course, the larger the number of cones, thecloser magnet 20 approximates an ideal magnet of the type where themagnetization changes continuously. It is noted that shell 21 iscomposed of seventy-two segments. For clarity, a ninety-degree portioncomposed of eighteen segments is not shown in FIG. 1.

Cones 31-39 are segmented along equally spaced meridians. It can be seenthat cones 31-39 are each comprised of eight similar segments (twosegments of each cone are not shown). Although cones 31-39 areillustrated as being segmented into eight segments each, they maycomprise a fewer or greater number of segments: the greater the numberof segments, the closer shell 21 will approximate an idealconfiguration.

The magnetization in each of the segments of cones 31-39 is uniform,i.e. constant in magnitude and direction. However, the magnetizationfrom segment to segment varies in direction with the average polar angleof its cone. Arrows M1, which are shown in FIG. 1 on the faces of thesegments, represents the direction and intensity of the magnetizationfor that particular segment.

As is well known in the art, shell 21 is capable of producing ahigh-intensity magnetic field of substantial uniformity in cavity 22.This field H is directed downwardly in a direction parallel to thevertical polar axis as viewed in FIG. 1. Specifically, the cavity fieldH of a "magic sphere" like shell 21 is: ##EQU1## where: B_(R) is themagnet remanence;

r_(o) and r_(i) are respectively the outer and inner radii of the "magicsphere;" and

H, the cavity field, is parallel to the polar axis.

Field H is augmented in accordance with the present invention byintroduction of the spherical magnetic insert 23. Insert 23 ismagnetized uniformly as indicated by arrow M2 in FIG. 1. The uniformmagnetization of insert 23 is directed opposite to the direction ofmagnetic field H. Essentially, insert 23 asserts a demagnetization fieldin the excess regions of cavity 22 such that the working field in tunnel26 will be increased by substantially B_(S) /3, where B_(S) is theremanence of insert 23 and 1/3 is the demagnetization coefficient of asolid sphere.

To illustrate the degree of field augmentation that is possible, assumethat shell 21 has an outer radius r_(o) of 3.3 cm and that both shell 21and insert 23 are made of the same high-remanence material, such asNd-Fe-B which has a magnetic remanence in excess of ten kilogauss. Inthis case, with a ratio of outer radius to inner radius (r_(o) /r_(i))equal to 3.32 for shell 21, a cavity field H of about 16.0 kilo-oersteadis produced by shell 21. Cavity field H is downwardly directed parallelto the vertical polar axis as viewed in FIG. 1. A working magnetic fieldof 19.3 kilo-oerstead will be produced in tunnel 26. This represents a17% increase in the size of the working field at a cost of 4.0 cm³(2.7%) increase in material volume represented by the addition of insert23. To obtain the same size working field without insert 23, i.e. bymaking shell 21 thicker, the cost is a 168 cm³ (112%) increase inmaterial volume and mass of shell 21. This technique offers an evengreater field gain when applied to a cylinder ("magic ring") because ofits larger demagnetization coefficient of 1/2.

FIG. 2 illustrates a segmented, cylindrical magnet 40 shaped as a "magicring." Magnet 40 has a plurality of segments that are nested to form acylindrical shell 41 with a concentric cylindrical cavity 44.Disregarding co-planar access ports 45 and 46, the segments are allsimilarly shaped. Also, each segment is uniformly magnetized M1 in aplane perpendicular to the cylindrical axis of magnet 40. From segmentto segment, the variation in the direction of magnetization M1 is twicethat of a polar angle where the cylindrical axis and the polar axis passthrough the center of ports 45 and 46. Cavity field H, a substantiallyuniform magnetic field, is the result of shell 41.

A cylindrical magnetic insert 48, composed of two spaced half-cylinders,is located in cavity 44. Insert 48 has a narrow, planar tunnel 49 thatis aligned with the ports 45 and 46. Access ports 45 and 46 are narrow,co-planar gaps that lie in the plane of the longitudinal axis ofcylindrical magnet 40. Ports 45 and 46 pass through the center ofopposed split segments 57 and 58. The plane of access ports 45 and 46 isaligned parallel to cavity field H. Magnetization M2 of insert 48 isdirected opposite to that of cavity field H. The working field, which isthe sum of the combined effects of shell 41 and insert 48, is located intunnel 49 and is directed in the same direction as cavity field H.

In addition to the augmentation of the working field produced by inserts23 and 48, there is also an augmentation of the external dipole fielddue to the large dipole moments of these inserts. In those cases whereflux confinement is important, substantially all external dipolar fieldscan be canceled by an oppositely directed dipole field via the additionof a uniformly magnetized outer shell.

FIG. 3 illustrates a spherical magnet 20' that is composed of sphericalshell 21 and insert 23 of FIG. 1, plus a uniformly magnetized outershell 60 that surrounds inner shell 21. Shell 60, being uniformlymagnetized, has no interior field and, therefore, has no effect on theworking field in tunnel 26. Shell 60 is uniformly magnetized M3 in adirection opposite to magnetization M2. As such, shell 60 produces anexterior dipole field that is opposite to the external dipole fieldproduced by insert 23. The intensity of the external dipole field ofshell 60 may be readily tailored by those skilled in the art so as tocancel out the external field produced by insert 23.

Moreover, the shape and direction of an access hole, bore through theconcentric shells to the cavity, can be adjusted by adding additionaluniformly magnetized shells having a predetermined polarity anddirection with respect to the inner shell. The direction and magnitudeof any additional uniformly magnetized shell may be readily determinedby those skilled in the art so as to provide the ability to bore anaccess hole of a desired shape and direction while preserving the cavityflux and the negate the external field.

FIG. 4 illustrates an equatorial spherical magnet 70 that is composed ofspherical shell 71 having a concentric cavity 73 and a spherical magnetinsert 72 located in cavity 73. Co-equatorial access ports 75 and 76pass through shell 71 on opposite sides of cavity 73 perpendicular tothe polar axis of the shell. A tunnel 74, which passes through thespherical center of insert 72, is aligned with access ports 75 and 76.Ports 75 and 76 and tunnel 74 form a narrow, cylindrical hole ofsufficient size to permit proper utilization of a working field which islocated in tunnel 74. Magnetization M2 of insert 72 is directed in linewith that of cavity field H. As such, the working field is the sum ofthe combined effects of shell 71 an insert Magnetization M2.

The working field of spherical magnet 70 may be enhanced even greater byusing a passive magnet (not shown), such as iron, as insert 72. Passivemagnets such as iron have magnetic properties for enhancing a workingfield in an equatorial structure. Even though a passive magnet does notcreate a magnetic field itself, it behaves as if it does create a fieldwhen placed in the presence of another field because its differentialpermeability is equal to one. For optimum enhancement, however, apassive insert 72 must be saturated by shell 71. In fact, when shell 71is magnetized such that it saturates the iron insert, the working fieldwill be enhanced almost twice a much as with a permanent magnet insert.This quality is due to iron having almost twice as much remanence as apermanent magnet.

Although enhancement will be significantly reduced, a non-saturatedpassive insert can still enhance the working field of a magic spherehaving a planar hole passing through the cavity bisecting the insert andsphere into two half-spheres. As such, the embodiment of FIG. 4 may bemodified to have a passive magnet insert instead of insert 72.

As in the spherical magnet of FIG. 1, all external dipole fields causedby insert 72 can be canceled by the addition of uniformly magnetizedouter shells (not shown in FIG. 4).

FIG. 5 illustrates an equatorial "magic ring" 80 that is composed ofcylindrical shell 81 having a concentric cavity 83 and insert 82. Insert82 is a magnetic material having a narrow, planar tunnel 84 that isaligned with ports 85 and 86 to form a plane perpendicular to cavityfield H. Magnetization M2 of insert 82 is directed in the same directionas that of cavity field H. Cavity field H is sum of the combined effectsof shell 81 and insert 82.

FIG. 6 illustrates an equatorial cylindrical magnet 90 having the sameproperties as that disclosed in FIG. 5, above, except that insert 92 isa passive magnet (i.e.: iron) rather than a permanent magnet. Dependingon the cavity field strength H created by shell 91, passive insert 92may enhance cavity field H even greater than a permanent magnet insertof similar size. This is due to the fact that a passive insert has adifferential permeability equal to one and twice the remanence of apermanent magnet insert. Consequently, passive insert 92 must besaturated by shell 91 to provide optimum enhancement of cavity field H.If insert 92 is not saturated, the cavity field will still be enhancedbut at a significantly reduced level.

Obviously many other modifications and variations of the presentinvention are possible in the light of the above teachings. For example,the inventive technique may be readily applied to a variety of othermagnetic flux sources. Those skilled in the art will find it obvious, inthe light of the above teachings, to employ this technique whendesigning toroidal magnets or hemispherical magnets. Further, thistechnique may be readily applied to cavities in magnetic structures ofother than spherical or cylindrical shape. In such cases, however, theworking field would vary in strength along the structural axis. Thiswould be useful for applications where field strength is all importantand uniformity is not important, e.g. in some Faraday rotators. It istherefore to be understood, that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed.

What is claimed is:
 1. A permanent magnet structure with an augmentedmagnetic field comprising:a spherical shell of permanent magnet materialforming a hollow spherical concentric cavity in which said shellproduces a magnetic field having a predetermined magnitude and adirection aligned with said shell's polar axis, said structure having anaccess port that passes through said shell along said polar axis andcommunicating with said cavity; and a spherical field-enhancing insertpermanently magnetized in a predetermined direction located in saidcavity adjacent to and concentric with the inner-surface of said shell,said insert having a tunnel communicating with said access port, saidinsert positioned within said cavity so as to enhance said cavitymagnetic field without obstructing said access port communicating withsaid cavity.
 2. The magnet of claim 1 wherein said access port and saidtunnel form a narrow cylindrical hole that passes through said magnetalong said polar axis.
 3. The magnet of claim 2 wherein said shellcomprises a plurality of magnetic segments and wherein each said segmentis uniformly magnetized.
 4. The magnet of claim 3 wherein said shell ismagnetized with a uniform magnitude and an orientation that varies as afunction of an average polar angle of said segments.
 5. The magnet ofclaim 4 further comprising an outer shell of magnetic material beinguniformly magnetized in a direction opposite to the magnetization ofsaid insert.